? Th9 cells in immediate hypersensitivity T helper subsets regulate inflammatory diseases, including the development of allergic lung inflammation associated with asthma. The development of T helper subsets in controlled by a network of transcription factors that promote the expression of cytokines and other genes associated with the ability of a T helper cell to promote disease. This proposal focuses on the most recently identified subset of T helper cells, Th9 cells that differentiate in response to TGF? and IL-4. In our preliminary data we have identified a requirement for Th9 cells for mast cell accumulation and function in allergic airway inflammation. Interestingly, we have identified a role for Th9 cells in facilitating immediate hypersensitivity responses. In the two Aims in this proposal we address basic questions that are critical for understanding Th9-dependent mast cell activity. In the first aim we will define the requirement for Th9 cells in immediate hypersensitivity using allergen sensitization and passive transfer models of IgE-dependent mast cell function. In the second Aim we will determine how Th9 cells regulate mast cell accumulation and activation, and determine which aspect is required for Th9-dependent mast cell function. Together, these Aims will define new mechanisms involved in Th9-dependent acute allergen responses. These Aims will elucidate new pathways in acute and chronic allergic responses and highlight potentially new pathways for intervention.